This paper presents research results showing how spraying distance affects the bonding strength, porosity, microhardness, and deposition efficiency of HVOF-sprayed NiCrAl coatings. The coatings examined were deposited on quenched 1045 steel substrates at spraying distances of 340, 360, and 380 mm. At the optimal standoff distance of 360 mm with kerosene and oxygen flow rates at 22 L/h and 850 L/min, respectively, NiCrAl coatings were achieved with a bonding strength of 66 MPa, microhardness of 418.56 HV 0.3 , porosity of 1.1%, and deposition efficiency of 65.3%.

C-BNp/NiCrAl composite coating was deposited by cold spraying using a mechanically alloyed composite powder. To modify coating microstructure, especially the bonding at the interfaces between c-BN particles and NiCrAl alloy matrix, and bonding at the sprayed particle/particle interface, annealing treatment at series of temperatures in Ar atmosphere was carried out. The results show that a zigzag interface layer is formed at the interface between c-BN particle and NiCrAl matrix after annealing at 825°C for 300 min through reaction of c-BN with NiCrAl. It is also observed that the thickness of the interface reaction layer increases with the increasing annealing temperature. Moreover, the interface between spray particles and the plastic deformation ability of the cermet coating can be improved through post-spray annealing. Vickers microhardness test shows that the hardness decreases with increasing annealing temperature due to the reduction of work hardening effect and grain growth of NiCrAl alloy matrix resulting from recovery and recrystallization during annealing treatment.

cBN/NiCrAl nanocomposite coating was deposited by cold spraying using mechanically alloyed composite powders. To examine the thermal stability of coating microstructure, the nanocomposite coating was annealed at different temperatures from 750 to 1000°C. The microstructure of composite coatings was characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results showed that the nanostructure was retained in the coating when the annealing temperature was lower than 825°C which is 0.7 times of the melting point of NiCrAl matrix. The dislocation density significantly reduced when annealing temperature was higher than 750°C. The cBN particle growth became significant when the annealing temperature was higher than 825°C. The effects of crystal grain refinement and work-hardening strengthening mechanisms were quantitatively estimated as the function of annealing temperature. The effect of annealing temperature on the contribution of different strengthening mechanisms to coating hardness was discussed.

In this work, diamond-NiCrAl composite powder was prepared by mechanical alloying and cold sprayed to form an ultra-hard cermet coating. The effect of diamond content on the characteristics of the milled powder and the as-sprayed coating was investigated. The results show that the microstructure, particle size distribution, and grain size of the powder was significantly influenced by the ball milling process and that the microstructure of the spray particles was completely retained in the coating.

Amorphous coatings, despite their high strength and hardness and outstanding corrosion and wear properties, have been limited in application due to poor bonding strength and low impact resistance. This paper reviews the progress that has been made in that regard through the addition of ductile metals, ceramic particles, and polymer phases and through laminar structure design consisting of alternating amorphous and NiCrAl layers. Test results show that the composite amorphous coatings realized by the various methods exhibit significantly improved bonding strength and impact resistance along with their other superior properties.

High velocity oxy fuel thermal spray (HVOF) processing is one of the proven thermal spray technologies to develop high temperature oxidation and corrosion resistant coatings for various high temperature applications such as thermal power plants, aero turbines, and energy conversion systems. In the present investigation Nickel based coatings, viz. Ni-5Al, NiCrAl, and NiCrAlY-0.4wt%CeO 2 were sprayed on Fe based superalloy (Superfer 800) by HVOF spray process as the chosen superalloy finds extensive use steam boilers, furnace equipment, heat exchangers and piping in chemical industry The degradation behaviour of the coated superalloy was studied in the aggressive environment of 40%Na 2 SO 4 -60%V 2 O 5 at 900°C, using thermogravimetric technique, to understand the protective nature of the coatings under cyclic conditions. The coatings were found to be effective in increasing the resistance to the degradation in the given environment. Among the coatings investigated, NiCrAlY-0.4wt%CeO 2 coating was found to be more protective on the superalloy in the given aggressive environment. The corrosion mechanisms were elucidated through the X-ray diffraction (XRD) analysis and microstructural characterization techniques such as FE-SEM/EDAX analysis of the corroded products formed at high temperature.

The application of thermally sprayed coatings is often limited by their porosity as well as their low adhesion. A new approach for high energy beam surface treatment of thermally sprayed coatings is presented using simultanously a lower power level for continuous remelting and pulses of higher power in order to get coatings with good corrosion behaviour and improved adhesion. This technology is applied to titanium as well as to NiCr-coatings on steel substrates. The investigations prove that remelted thermally sprayed coatings with a smooth and dense surface layer, followed by a heat treated section of the coating and a reaction zone at the substrate/coating interface can be obtained. Therefore, a change in chemical composition of the coating is avoided, while at the same time the bond strength of the remelted coatings can be influenced by metallurgical reactions at the interface zone.

In a BRITE-EURAM Project gas shroud devices were developed for arc-wire and air plasma spraying processes to obtain cost effective coatings with high density, low defect contents and high corrosion resistance. To test the two devices FeCrAl, NiCrAl and NiCrTi coatings sprayed by shrouded arc were studied for boiler application; AMDRY 995 coatings sprayed by shrouded plasma were analysed for gas turbine blades. In this paper the results are presented of the extensive characterization performed by CESI on the coatings produced with the innovative techniques developed by the other partners. The coatings were sectioned and analysed by metallography and image analysis to determine oxide and porosity contents (total defect levels were less than 5%). Hardness and adhesion were measured following the ASTM standard procedures. On FeCrAl, NiCrAl and NiCrTi coatings sprayed by shrouded arc-wire technique both thermal cycling and corrosion tests were performed. A sulphidizing gaseous atmosphere was used to simulate boiler environment burning sulphur rich fuel as Orimulsion. The corrosion behavior after 1000h at 450°C resulted to be as good as that of an HVOF sprayed similar coating. Moreover coated tubes survived to thermal cycling tests without any delamination. Hot corrosion "Dean tests" were performed on AMDRY995 coatings sprayed by shrouded plasma onto two Ni-base superalloys (Ud 529 and In738). The environment was typical of hot parts in gas turbines; the corrosion resistance results after 1750h at 850°C were comparable to those typically obtained on the same material sprayed by vacuum plasma spray technique. The applicability of the shrouded plasma technique to the field of gas turbines was confirmed.

Thermal sprayed coatings often need to be removed for maintenance, repair or recycling and are accomplished by various conventional methods such as grinding, abrasive blasting and chemical removal processes. The coating removal for repair is typically done for expensive and/or very complex parts. Parts of turbines, equipped with functional coatings, are stripped for inspection and repair as well as old coatings are renewed. When removing a thermal sprayed coating, damage to the part or ablation of the substrate must be avoided. A new process of dry ice blasting alone as well as in combination with a laser beam in a hybrid process has been approved for the removal of thermal sprayed coatings in a project aided by the Federal Ministry of Economic Affairs and Employment via the Federation of Industrial Research Associations (AIF). The aim of the project is to obtain the properties for the comparison and benchmarking of coating removal processes. Comparative studies have been made with the water jet and the abrasive water jet process at the Institute of Materials Science of the University of Hanover.

The Ti 2 AlNb blade is used in high pressure compressor aero-engines to provide high thrust force at relatively light weight. A series of abradability tests was carried out on CuAlNi-graphite, NiCrAl-graphite, NiCrAl-bentonite, and NiCrFeAl-hBN abradable coatings rubbed against Ti 2 AlNb dummy blades with the maximum blade-tip velocity of 300 m/s at 500 °C. In consideration of the effects of an engine’s working conditions, some tests were conducted with incursion rate as the single variable. The scratched surfaces of the samples were observed by the stereoscopic optical camera, and a ratio of the blade wear to shroud incursion depth (IDR) was evaluated to characterize the abradability of coatings. The results show that NiCrAl-graphite and NiCrFeAl-hBN abradable coatings perform very well rubbed against the Ti 2 AlNb blade, and the blade-tip wear is not obvious after abradability tests.

Arc spraying has always been the most cost-effective way of thermal spraying metal alloys but oxide content and degradation of the alloy by loss of particular alloying elements have limited quality of the coatings. In this paper a process is described which greatly reduces degradation and improves coating density and oxide content. Corrosion behaviour both in aqueous and high temperature environments is markedly improved. For aqueous applications coatings of Inconel 625 were tested in a potentio-dynamic cell and by salt spray testing to evaluate both the inherent properties and the permeability of the coating and significantly improved behaviour was found in both cases. For high temperature corrosion, samples of FeCrAl were tested in air and in a sulphidising environment with and without thermal cycling. Coatings of NiCrAl and NiCrTi were also examined. The coating types and test regimes were aimed at specific practical applications such as fireside corrosion in boilers and waste incinerators, hot oxidation of flare stack burners etc. In the aqueous situation valves and process vessels are being examined as candidate applications for Inconel 625 coatings.

This paper reports morphological study of coatings microstructure performed on two selected reference abradable materials: AlSi-hBN and NiCrAl-Bentonite. Scanning Electron Microscopy (SEM) was used to obtain adequate micrographs for analysis. The porosity and non-metal particles morphology was assimilated to elliptical inclusions within the metallic matrix. Accordingly, they were described by means of three different geometrical parameters related to the shape, size and orientation distribution, allowing additional statistical analysis on the coatings.

Abradable coatings are located on the stationary parts of gas turbines, in front of blades, which cut a track in them. This has to be achieved with minimum wear of the blades, in order to control the over-tip leakage. These coatings are generally deposited by thermal spraying of composite powders comprising a metal base, a polymer filler generating porosities and a dislocator such as hBN. The very demanding properties are nowadays adjusted using rig tests, where samples are rubbed by the contact of a dummy, simulating actual working conditions in an aircraft engine. Several types of behaviour are usually described, but few numerical data are produced from these tests. Only the blade wear (or metal transfer) is generally measured. As the understanding of contact phenomena is fundamental for the development of predictive models allowing the design of more performing materials, a comprehensive characterization process of the rub path was developed. The study was based on a topological survey made by laser profilometry, giving three-dimensional maps. These maps were then processed by image analysis and several parameters were computed, like surface roughness and parameters giving information on the shape and orientation of the holes or grooves in the rubbed surface of the samples.

Abradable coatings are typically applied on the compressor section of gas turbines to reduce air leakage and increase compressor performance. In pursuit of engine efficiency, the service temperatures of the components are higher than before. The use of nickel-graphite coating in compressor applications in higher temperature environments diminishes the abradable property of the coating. In the current study, a series of abradable coatings were prepared with combustion and plasma spray methods and tested at gas turbine conditions. Coating microstructure, hardness, abradability, and erosion resistance was investigated and compared against conventional nickel-graphite coating. In addition, coatings were aged to mimic the aging cycle in industrial gas turbines and compared to as-sprayed coating properties.

In this work, Al/SiC composite coatings were deposited on the surface of aluminum alloys through atmospheric plasma spray. The effects of SiC volume in Al/SiC composite powders on the deposition behavior and the properties of the Al/SiC coatings were investigated. It was found that there were decarburization and oxidation during the deposition of pure SiC powders in the plasma flame. With the increase of SiC content, the deposition of the Al/SiC composite powder became more difficult through plasma spray. There were cracks between pure SiC and the bond coat on the Al alloy substrate resulting in poor adhesion between them. The hardness of the composite coating became higher with the increase of SiC contents. The Al/SiC(50:50) deposit with a thickness of 70 µm and a hardness of 369 Hv resulted in a strengthening and protection on the surface of Al alloy.

Chemical composition differences between the feedstock powder and the final coating of a series of composite abradable coatings were investigated. Graphite filler material mass distributions in the coating, overspray powder and burned power is calculated. A preliminary mechanism of graphite loss during the deposition process is established. It is found that the graphite content in the coating is significantly lower than that in the feedstock powder. Over 70% graphite in the feedstock powder is lost during the deposition process. Melting and shrinkage of the nickel shell of the nickel cladded graphite particle as flying through the flame, which resulted in the exposure of the graphite core to the flame and substrate, is the main reason for graphite loss and chemical composition change between the feedstock powder and the final coating. A random manner of particle structure transformation in the flame and its reactions with the spray environment is concluded as an important reason for the poor process repeatability of abradable coatings.

A high electromagnetic field (0.3 T-Teslas) was applied during the solidification of Ni-based alloyed splats. Ni, NiCr, NiCrAl, NiCrBSiFe powders were deposited over steel polished substrates using a flame spray and a plasma spray torch. A strong electromagnet was used to produce sufficient magnetic field to induce effects over the splats during solidification. A remarkable change in splat morphology and chemical segregation was identified specially in NiCrBSiFe and the other alloys. Optical microscopy, surface profilometry, and SEM images revealed changes in the regular cracking trends, splashing, and thickness of the splats. This experimental study discusses the possible explanations for this phenomena. The adherence of the coating is the main property to be analyzed with the goal of improving the mechanical interlocking, and therefore, adhesion by engineering the applied electromagnetic field.

NiCrAl/ZrO 2 -8Y 2 O 3 coatings deposited on SUS304 stainless steel and 45 carbon steel substrates were prepared by APS at different preheating temperatures, of which thickness exceeded 1mm. This study analyzed the coatings’ separation from different preheated substrates in the cooling process after spraying due to residual thermal stress. The Young’s modulus of the porous YSZ coatings was calculated and also measured by Knoop indentation methods for comparison purposes. The result indicated that the failure of porous thick YSZ coatings is mainly caused by the cracks nucleation, propagation and coalescence, which is related to the thermal-expansion coefficient difference between substrate and coatings, preheating temperature, porosity of coatings and so on. Due to their increased porosity, the porous and thick YSZ coatings had much lower calculated and measured Young’s modulus values than the sintered YSZ coatings.

In modern jet engines, the efficiency of the compressor stages is highly dependent upon the clearance between housing and rotating compressor blades. To control the over-tip leakage, abradable coating systems are applied on the housing. In the high pressure compressor they typically consist of a thermal sprayed multiphase material, comprising a metal matrix combined with a dislocator and/or a solid lubricant as well as a defined level of porosity. In this study, novel material systems have been sprayed via the plasma and flame spray process and compared to reference materials. Resulting microstructures have been analyzed as well as important coating characteristics evaluated, including coating hardness and erosion resistance. Furthermore rig tests were performed to analyse the coatings abradability behaviour under different operation conditions of the compressor.

This paper examines the potential of thermal spray coatings for manufacturing and repairing twin-screw rotors. The coatings evaluated were produced by atmospheric plasma spraying (APS). Tests show that 7% yttria stabilized zirconia (ZrO 2 7Y 2 O 3 ) coatings provide sufficient run-in characteristics and corrosion resistance. The coatings, along with a NiCrAl bond coat, were applied to various substrate materials.